Some interesting research at the University of Florida into the possible source of 'ball lightning' reports:

Using tethered rockets to trigger and direct lightning to the ground and through a wide variety of materials, lightning researchers at the University of Florida at Gainesville have managed to create some short-lived balls of fire that have a lot in common with the naturally-occurring floating balls of light.

...Some previous laboratory experiments have also managed to ignite floating wisps of silicon into something that resembles ball lightning. The actual lightning experiments duplicated that effect repeatedly, said Hill.

These experiments point to one possible explanation of ball lightning: that it isn't actually lightning at all. Rather, it is some material that has been vaporized and ignited by lightning. It burns briefly and then blinks out. But is it really ball lightning?

"What it demonstrates is that it is possible to produce luminous events at the surface that to an untrained observer looks like ball lightning," said Richard Orville, professor and director of the Cooperative Institute for Applied Meteorological Studies at Texas A&M University.